Fluid pressure apparatus

ABSTRACT

A variable-displacement pump or a variable-displacement motor having a smoothly adjustable servo-motor for displacement adjustment and a plurality of separate manually operable controls for adjusting the servo-motor to any one of a number of preselected displacements for the pump or motor. For adjustment of the servo-motor to an intermediate displacement position, a valve means is provided actuatable by the servo-motor at a position corresponding to a selected intermediate displacement, and a selecting valve co-operable with the valve means, said selecting valve having two settings, in one of which the valve means is rendered capable of controlling the pressure in a working volume of the servo-motor to arrest movement thereof substantially in the selected position, and in the other of which the valve means is rendered incapable of controlling the pressure in the working volume.

[45] May 15,1973

I54] FLUID PRESSURE APPARATUS [75] Inventor: Kenneth Raymond Boydell,Tewkesbury, England [73] Assigneez Dowty Technical Developments Limited,Brockhampton, England [22] Filed: Apr. 30, 1971 [21] App]. No.: 138,874

Primary Examiner-William L. Frceh A tlorney Young & Thompson [57]ABSTRACT A variable-displacement pump or a variable-displacement motorhaving a smoothly adjustable servo-motor for displacement adjustment anda plurality of separate manually operable controls for adjusting theservo-motor to any one of a number of preselected displacements for thepump or motor. For adjustment of the servo-motor to an intermediatedisplacement position, a valve means is provided actuatable by theservo-motor at a position corresponding to a selected intermediatedisplacement, and a selecting valve cooperable with the valve means,said selecting valve having two settings, in one of which the valvemeans is rendered capable of controlling the pressure in a workingvolume of the servo-motor to arrest movement thereof substantially inthe selected position, and in the other of which the valve means isrendered incapable of controlling the pressure in the working volume.

13 Claims, 2 Drawing Figures [51] Int. Cl ..F0lb 13/04 [58] Field ofSearch ..91/506; 417/222; 60/52 VS [56] References Cited UNITED STATESPATENTS 2,415,603 2/1947 Muller et a]. ..60/52 VS 3,256,830 6/1966Budzich ..91/506 3,366,064 1/1968 Stephens et a] "60/52 VS 3,528,2439/1970 Cryder et al ..60/52 VS 3,601,986 8/197] Becker ..60/52 VSPATENTEDMYI 5W 3.732785 SHEET 1 Br 2 I N V E NTOR Kim/5 m fla /1404 0 0you L ATTORNEYS PATEHILURMISEYS ,7 5

SHEET 2 OF 2 lNVENTOR' ATTORNEYS FLUID PRESSURE APPARATUS This inventionrelates to a variable-displacement pump and to a variable-displacementmotor having a displacement adjusting servo-motor. A smoothly adjustableservo-motor is well-known and will enable infinitely-variabledisplacement adjustment of the pump or the motor to be obtained. Formany purposes it is desirable to have one or more separatemanuallyoperable controls whereby a preselected or any one of a numberof preselected displacements may be readily obtained and it is the mainobject of the present invention to provide such an arrangement. Afurther object of the invention is to provide such a control meansoperable at a remote position having regard to the pump or motor.

The present invention broadly comprises a variabledisplacement pump or avariable-displacement motor having a fluid-pressure-operated servo-motorcapable of adjusting pump or motor displacement from one limit toanother limit by the supply of fluid at pressure to or from a workingvolume of the servo-motor, a valve means actuatable by the servo-motorat a position corresponding to a selected displacement between thelimits, and a selecting valve co-operable with said valve means, saidselecting valve having two settings, in one of which the valve means isrendered capable of controlling the pressure in the working volume toarrest the movement of the servo-motor substantially in the selectedposition and in the other of which it is rendered incapable of socontrolling the pressure.

A further feature of the invention comprises a variable-displacementpump or a variable-displacement motor having a hydraulically-operatedservo-motor capable of moving to adjust the pump or the motordisplacement by the supply of liquid to or from a working volume in theservo-motor under the control of a main selecting valve having twosettings in one of which the working volume is fed with arestricted flowat pressure to urge the servo-motor to one displacement limit and in theother of which the working volume is connected to low pressure tofacilitate servo-motor movement to the other displacement limit under arestoring force, an auxiliary valve means actuatable by the servo-motorat a position corresponding to a selected displacement between thelimits, and an auxiliary selecting valve cooperable with the auxiliaryvalve means and having two settings in one of which the valve meansprovides a vent to a low pressure zone such that at the selectedposition when the main selecting valve is in said one position; asubstantial pressure change in the working volume is caused to occur tothe extent to arrest the servo-motor substantially in the selectedposition, and in the other of which the valve means is renderedincapable ofcarrying liquid flow.

By this invention, when the selecting valve is moved ing volume at theparticular pump or motor displacement intermediate the limits.

A plurality of valve means may be provided at differing displacementsbetween the two limits, each valve means having a selecting valveassociated therewith whereby the servo-motor may be stopped at theposition of any one of the valve means by the appropriate selection ofsettings of the selecting valves.

The servo-motor may draw its actuating fluidfrom a supply of restrictedflow rate and the action of the or a valve means may merely be toconnect or dis-connect the restricted supply to or from a low pressurezone. For use with a variable-displacement pump, the delivery of thepump itself may supply fluid through a restrictor to form the supply ofrestricted flow rate provided that the range of adjustment between thesaid limits does not include zero pump displacement.

A relay valve may be provided movable in accordance with movement of theselecting valve for rendering the said valve means operative orin-operative.

The relay valve may comprise a relay piston slidable in a relay cylinderagainst spring loading, the relay cylinder including a plurality ofports connected to the servo cylinder ports, and the relay piston beingadjustable against spring loading to selectively cover or uncoverdiffering numbers of the ports by means of selective pressure adjustmentthereof under the control of the selecting valve.

Two embodiments of the invention for use with a variable-displacementpump will now be particularly described with reference to theaccompanying drawings, in which FIG. 1 is a diagrammatic illustration ofthe first embodiment, and

FIG. 2 is a diagrammatic illustration of the second embodiment.

Referring initially to FIG. 1, the variabledisplacement pump 1 is of anykind in which displacement is variable by movement of a member such asthe lever 2 and is also of the kind which will tend to move on its owntowards the minimum displacement position. An example of such a pump isa swash-plate pump in which the angular setting of the swash-plate isadjustable to vary displacement. A piston-and-cylinder servomotor 3 isconnected to the lever 2 by means of a link 4. The servo-motor comprisesafixed cylinder 5 having a piston 6 slidable therein being connected tothe link 4 by means of a connecting rod 7. The pump is arranged to drawliquid from a low pressure reservoir 8 and to deliver it to a deliverypipe 9 which may be connected to any suitable hydraulic load.

Within the cylinder 5 a-stop 11 is provided against which the piston 6engages to define the minimum displacement limit of the pump. Liquid atpressure from the delivery pipe 9 is connected through a restrictor l2and pipe 13 to the right-hand end of the cylinder 5 which includes thestop ll. The pipe 13 is also connected by way of main selecting valve 14to a pipe 15 returning to the reservoir 8.

The valve means comprises three spaced ports 16, 17 and 18 in the wallof the cylinder 5. Ports l6 and 17 each comprise an auxiliary valvemeans. The port 16 is connected through an auxiliary selecting valve 19to the pipe 15 and the port 17 is connected through an auxiliaryselecting valve 21 to the pipe 15. The port 18 is connected directlythrough a pipe 22 to the pipe 15 and a branch pipe 23 also connects theextreme lefthand end of the cylinder 5 to the pipe at a position whereit cannot be closed by the piston 6. The three selecting valves 14, 19and 21 are each comprised by a simple manually-operable valve havingopen and closed positions.

In operation of the FIG. 1 embodiment, the pump 1 is suitably driven andliquid is drawn from the reservoir 8 and delivered to the pipe 9 fromwhence it will pass to the load. Liquid at pressure also flows throughthe restrictor 12 to the right-hand end of cylinder 5. If minimumdelivery flow only is required from the pump, the first control valve 14is opened to reduce the pressure at the right-hand end of cylinder 5 toreservoir pressure. Then quite irrespectively of the positions of thevalves 19 and 21, the piston 6 will remain in the minimum displacementposition. If the pump is not capable on its own of moving to the minimumdisplacement position, then it may include a device such as a springcapable of exerting a constantly acting force tending to move thedisplacement to the minimum position. In order to obtain an increasedflow rate from the pump, valve 14 is closed and valve 19 is opened. Thepressure will then build up at the right-hand end of the cylinder 5 tourge the piston 6 to the left to increase pump displacement until thepiston opens the port 16 to the working volume. At this point liquidwill flow through the port 16 and the valve 19 to reservoir takingliquid from the working volume to the extent that the pressure remainingis just sufficient to maintain the pump at a displacement adjustmentcorresponding to the position of the port 16. Assume now that a furtherincrease in delivery flow rate is required. The valve 19 is then closedand the valve 21 is opened. The servo piston will then travel up to theposition where it just connects the port 17 to the working volume to theextent that liquid escaping from the working volume through the valve 21reduces pressure so that the piston is just retained in position at theport 17 thus giving the required increased displacement. in order toobtain maximum displacement the three valves l4, l9 and 21 are allclosed and the piston 6 will then move to the position where the port 18is connected to the working volume, flow being permitted to the extentthat the pressure retained within the working volume is sufficient tohold the pump at its maximum displacement. The port 18 becomeseffectively the maximum limit of pump displacement and provided thatvalves 19 and 21 remain closed, the opening and closing of valve 14 willmove the servo piston 6 respectively to minimum and maximum displacementpositions of the pump.

it will be apparent that. the piston 6 can be arranged to adjust thepump to any of four particular displacement adjustments depending on thesettings given to the three selecting valves l4, l9 and 21.

It will also be apparent that the valve means formed by the ports 16, 17and 18 may be replaced by alternative valve means mechanically connectedfor movement with the servo piston 6.

Reference is now made to the second embodiment of the invention shown inFIG. 2. The variabledisplacement pump 31, which again is capable on itsown of moving to the minimum displacement position, is connected fordisplacement adjustment through the medium of a lever 32. A servo-motorcomprising a servo-cylinder 33 and a servo piston 34 act through apivoted link 35 to control the lever 32 for displacement adjustment. Thepump 31 draws liquid through pipe 36 from a reservoir 37 and deliverssuch liquid to pipe 38 under pressure. The selecting valve assembly39,which may be remotely placed relative to the pump 31, comprises fourvalve units 41, 42, 43 and 44 bolted together to form a completeassembly. The valves 41, 42 and 43 may each be of the kind shown in my[1.5. patent application Ser. No: 132478 filed in April 1971 The fourvalves 41, 42, 43 and 44 operate on the well-known open-center principleand the delivery passage 38 passes through a passage '45 Within thevalve units. Each of the valve units includes an operating handlerespectively 46,47,48 and 49. Each handle is capable of movement fromthe neutral position into a forward or a reverse position. When thehandle is in its neutral position, the open-center passage through thevalve unit is in an open unrestricted condition. When a handle is movedto either a forward or a reverse position the open-center passage isclosed and is connected to the supply and return connections of a load.The valves 41 to 44 respectively are connected to loads 51, 52, 53 and54 which for convenience are indicated as doubleacting hydraulic jacks.

The valves 41, 42 and 43 each hasan auxiliary opencenterfpassage55extending from the reservoir connection 36. The function of theauxiliary open-center passage 55 is for control of the servo-motor in amanner to be described. The valve 41 is the main selecting valve andvalves 42 and 43 are each an auxiliary selecting valve The servocylinder 33 and piston 34 are arranged as described in US. Pat. No.3,667,867. In particular, the servo piston 34 is mounted in the servocylinder 33 to enclose a working space 56 between the piston and acontrol unit 57 mounted in the servo cylinder. The control unit is fedwith liquid from the pump delivery connection 38 into the passage 58 andincludes structure which is responsive to the pressure of this liquid tocontrol the axial movement ofa control rod 59 which extends from thecontrolunit into a bore 61 within the piston. The control rod is locatedby a loose pin and hole connection 62 within the piston to be capable oflimited lost motion relative thereto, such limited lost motion adjustingthe opening of a vent port 63 within rod 59 from the working space 56 tothe interior of the pump casing which in turn is. connected to the tank37. Under normal conditions of operation the rod 59 will be urged to theleft limit of its movement so that the port 63 is maintained closed.High pressure liquid from the connection 58 has access to the workingspace 56 a relay valve 65 is provided. This valve may be formed as shownin the same structure which accommodates a servo-motor or it may beseparately formed. The relay valve comprises a relay cylinder 66 withinwhich a relay piston is slidably mounted being urged to the rightagainst a stop 68 by means of a compression spring 69 also within thecylinder. The right-hand end of cylinder 66 forms working space 60.Three ports 71, 72 and 73 open in axially spaced relation both in theservo cylinder 33 and into the relay cylinder 66. The minimumdisplacement position of the servo piston 34 as shown, i.e. completelyto the right, corresponds with the minimum displacement position of therelay piston 67 also completely to the right. In these positions theservo piston 34 completely covers the ports 71, 72 and 73, but the relaypiston leaves these ports completely uncovered. The space in relaycylinder 66 to the left of piston 67 is connected to reservoir throughpipe 36. A passage 74, uncontrolled by piston 34 extends from workingspace 56 to the end of the auxiliary open-center passage 55 in theselecting valve assembly remote from the pipe 36. The pipe 74 is alsoconnected by means of a restrictor 75 into the cylinder 66 on the leftof piston 67 and also by a restrictor 76 to the right-hand end of thecylinder 66. The restrictor 75 is closed by the piston 67 on initialmovement from its right-hand position. It will be seen that furthermovement of the relay piston 67 to the left will successively close theports 71, 72 and 73. Three ports 77, 78 and 79 open into the cylinder 66at a position to the right of the ports 71, 72 and 73 so as to be closedby piston 67 when in its right-hand position. The ports 71 and 72opening into cylinder 33 each form an auxiliary valve means. The axiallength of the piston 67 is such that when the piston moves to the left,it may occupy any of three controlling positions in which one of theports 77, 78 or 79 are just opening to the working space 60. In thesethree positions, the lefthand edge of piston 67 stops short respectivelyof the ports 71, 72, and 73.

The port 77 connects to a pipe 81 through check valve 82 into theauxiliary open-center passage 55 at a position between valve units 42and 43. The port 78 connects to a pipe 83 through a check valve 84 tothe auxiliary open-center passage 55 at a position between valve units41 and 42. The port 79 connects through a check valve 85 to theleft-hand end of cylinder 66 for permanent connection to reservoir. Thecheck valves 82, 84 and 85 are all arranged such as to permit flow onlyin a direction away from the working space 60.

In mounting the selecting valve assembly remotely from the pump, thepipes 74, 81, 83 and 86 may be quite small diameter pipes. The pipe 86connects the auxiliary open-center passage 55 back to the reservoirconnection 36. The pipe 38 which leads to the opencenter passage 45 andthe return flow pipe 87 connecting the open-center passage 45 back toreservoir are both of substantial size since these will carry the fulldelivery flow from the pump. In operation, assume that it is desired tooperate the load 54 which is a load requiring only a very small flowrate. The handle 4? is moved from its neutral position closing theopen-center passage 45 and directing pump delivery to the load 54. Theminimum displacement position of the'servo piston 34 as shown ensuresthe minimum delivery flow rate for operation ofload 54. In this-minimumdisplacement position liquid at delivery pressure enters the workingspace 56 of the servo through restrictor 64 and escapes through thepassage 74. The passage 74 is formed by a long thin pipe which itselfpossesses a restrictive effect. Therefore leakage liquid leaving theworking space 56 of the servo has two paths back to tank, the firstbeing through restrictor 75 and the second being through pipe 74 andauxiliary open-center passage 55. The

combined effect of these two flows reduces the pres sure in workingspace 56 to the extent that the servo piston 34 cannot move to increasepump stroke.

Assume now that it is desired to operate the load 53 and the handle 48of auxiliary selecting valve 43 is moved from the neutral position,handle 49 then being moved back to the neutral position. Movement ofhandle 48 from the neutral position closes the main opencenter passage45 and connects the load 53 to receive the main pump output. Movement ofthe handle 48 also closes the auxiliary open-center passage 55 thusclosing the flow through pipe 74. The leakage flow rate from the workingspace 56 is therefore reduced and the pressure in working space 56 willtherefore rise urging the piston 34 to the left to increase pumpdisplacement. The increase in pressure in working space 56 is alsoaccompanied by an increase in pressure in the pipe 74 which fed throughrestrictor 76 into working space 60 will raise pressure to move relaypiston 67 to the left against spring load 69. The relay piston will moveuntil port 77 opens and in this position restrictor vwill beclosed. Atthe opening of port 77, liquid will be vented from working space 60through pipe 81 and auxiliary open-center passage 55 to drain. Theopening of port 77 will thus be automatically adjusting to a positionwhere port 77 is opened only to the extent to maintain a sufficientpressure in working space 60 to balance the load of springv 69. Closureof restrictor 75 will cause further rise of pressure in working space 56to urge the piston 34 further to the left to uncover the auxiliary valvemeans formed by port 71. Further leakage flow will take place throughport 71 to reduce pressure in working space 56 to the extent that piston34 will be located accurately in a position where port 71 is justopened. The delivery flow rate from the pump through the load 53 willthen be in accordance with the displacement selected by the new positionfor servo piston 34.

Assume now that it is desired to operate the load 52 which requires agreater flow rate than the load 53. The handle 48 is put into itsneutral position and the handle 47 of auxiliary selecting valve 42 ismoved from its neutral position to close the open-center passage 45 invalve unit 42 to connect the pump delivery to the load 52. Movement ofthe handle 47 will also close the auxiliary open-center passage in valveunit 42 so that flow is prevented through both of the'long thin pipes 74and 81. Leakage is thus prevented through both these pipes and thepressure in working space 60 will rise to move relay piston 67 to theleft so as just to open port 78 through which liquid may escape todrain. The. opening is automatically adjusted so that the reducedpressure in working space 60 just balances the load of spring 69. Atthis position for the piston 67, the restrictor 75 and port 71 areclosed, ports 72 and 73 being open. The closure of port 71 will causepressure to rise in working space 56 which will urge servo piston 34 tothe left to increase pump delivery. Movement to the left will occuruntil the piston starts to open at the auxiliary valve means formed byport 72 and leakage flow through this port to drain will then establisha reduced pressure in working space 56 to balance the load on the servopiston 34.

Assume now that it is desired to operate the load 51 which requiresmaximum delivery from the pump. The

handle 47 is replaced to neutral and the handle 46 of the main selectingvalve 41 is moved from neutral closing the main open-center passage 45in the valve unit 41 and thus connecting pump delivery'to the load- 51.Movement of handle 46 will also close the auxiliary open-center passage55 thus preventing leakage flow through any of the pipes 74, 81 and 83.The pressure in working space 60 will therefore rise causing piston 67to move to the left until port 79 is opened slightly.

The leakage flow through port 79 will reduce pressure in working space60 to the extent to balance the load of spring 69. At this position ofthe relay piston 67, restrictor 75 and ports 71 and 72 are closed. Thepressure in working space 56 will therefore rise causing servo piston 34to move to increase pump displacement, such movement taking place untilpiston 34 begins to open the port 73. The leakage flow through port 73will then reduce pressure in working space 56 to the extent that theservo piston will be located at the maximum displacement limit.

It will be seen that the use of the relay valve enables control of theservo piston 34 to be obtained by virtue of controlled leakage from theworking space 56, such leakage whilst being small in comparison withpump delivery flow rates, nevertheless, itself being sufficientlysubstantial as to cause large pressure drops if it were to flow throughthe thin pipes 74, 81, 83 or 86. The relay valve 65 requires aconsiderably smaller leakage flow rate to adjust its position, and thusthe servo piston 34 and the leakage flow control of the relay valve maybe accomplished by the selecting valve assembly 39 through the use ofthe long thin pipes 74, 81, 83 and 86.

Whilst FIG. 2 illustrates the use of the open-center type of selectingvalve in conjunction with stepped adjustment of the servo piston and theuse of a relay valve, it will be appreciated that within the broad scopeof the present invention the relay valve 65 could be omitted and thepipes 81 and 83 could be connected directly to the ports 71 and 72 todirectly control pressures in the working space 56. However, in thiscase, the selecting valve assembly could not be remotely located sincethe restrictive effects of the pipes 74, 81, 83 and 86 would prevent anysubstantial adjustment of pressure in the working space 56 when thepiston 34 uncovered the ports 71 and 72. I

The control unit 57 may operate in any of the manners disclosed in U.S.Pat. No. 3,667,867, the control effected thereby being a maximum limitcontrol in which case the movement of the servo piston 34 to increasedisplacement would open the vent port 63 if the delivery of the pumpwere greater than that demanded by the control unit 57. If the pumpdelivery is less than that dictated by the control unit 57 the movementof the valve rod 59 in the piston 34 would be to the limit of the lostmotion 62 so as to close the vent port 63. Assume for example that thecontrol unit 57 is intended to control the pump so as to provide asubstantially constant maximum driving torque. If the driving torque forthe pump tends to rise above this maximum value, then the servo piston34 would tend to move relative to the control rod 59 to open the vent 63and thus to overridingly reduce pressure in the working space 56 so thatthe maximum driving torque could not be exceeded.

l claim:

1. A variable-displacement pump or a variabledisplacement motor having ahydraulically-operated servo-motor capable of moving to adjust the pumpor the motor displacement by the supply of liquid to or from a workingvolume in the servo-motor under the control of a main selecting valvehaving two settings in one of which the working volume is fed with arestricted flow at pressure to urge the servo-motor to one displacementlimit and in the other of which the working volume is connected to lowpressure to facilitate servo-motor movement to'the other displacementlimit under a restoring force, an auxiliary valve means actuatable bytheservo-motor at a position corresponding to a selected displacementbetween the limits, and an auxiliary selecting valve co-operable withthe auxiliary valve means and having two settings in one of which thevalve means provides a vent to a low pressure zone such that at theselected position when the main selecting valve is in said one position,a substantial pressure change in the working volume is caused to occurto the extent to arrest the servo-motor substantially in the selectedposition, and in the other of which the valve means is renderedincapable of carrying liquid flow.

2. A variable-displacement pump or a variabledisplacement motor having afluid-pressure-operated servo-motor capable of adjusting pump or motordisplacement from one limit to another limit by the supply of fluid atpressure to or from a working volume of the servo-motor, a valve means'actuatable by the servomotor at a position corresponding to a selecteddisplacement between the limits, and a selecting valve cooperable withthe said valve means, said selecting valve having two settings, in oneof which the valve means is rendered capable of controlling the pressurein the working volume to arrest movement of the servo-motorsubstantially in the selected position, and in the other of which it isrendered incapable of controlling the pressure.

3. A variable-displacement pump or motor as claimed in claim 2, whereinthe servo-motor is of the piston-and-cylinder type, and the valve meansis a port in the cylinder itself opened or closed by movement of thepiston relative thereto at the position of the port.

4. A variable-displacement pump or motor as claimed in claim 3,including a plurality of valve means at spaced displacement positionsbetween the two limits, each valve means having a selecting valveassociated therewith, whereby the servo-motor may be stopped at theposition of any one of the valve means by appropriate selection of thesettings ofthe selecting valves.

5. A variable-displacement pump or motor as claimed in claim 4, whereinthe working volume is fed with liquid at pressure from a supply ofrestricted flow rate and the action of the valve means is to connect ordis-connect the restricted supply to or from a low pressure zone inaccordance with movement of the servopiston.

6. A variable-displacement pump as claimed in claim 5, wherein thedelivery of the pump itself, fed through a restrictonforms the saidsupply of restricted flow rate for feeding to the said working space.

7. A variable-displacement pump or motor as claimed in claim 2,including a relay valve movable in accordance with movement of theselecting valve for rendering said valve means operative orin-operative.

8. A variable-displacement pump or motor as claimed in claim 7 whereinthe relay valve comprises a relay piston slidable in a relay cylinderagainst spring loading, the relay cylinder includes a plurality of portsconnected to the servo cylinder ports, and the relay piston isadjustable against spring loading to selectively connect and dis-connectdiffering numbers of the ports I by means of selective pressureadjustment under con said relay cylinder uncovered progressively duringmovement of said relay piston to connect to a working space in saidrelay cylinder, a restricted supply of pressure liquid connected to saidrelay working space, and said selecting valve comprises a plurality ofmanually operable valves for selectively connecting said selecting portsto drain, the relay valve moving to a position to partially close aselecting port connected to drain by a manually operable valve in orderto adjust pressure in the relay working space to meet the spring load atthat position.

10. A variable-displacement pump or motor as claimed in claim 9, whereinthe manually-operable valve means are connected in series in a passageextending to drain and the selecting ports are connected to said passageat positions intermediate said manuallyoperable valves.

11. A variable-displacement pump or motor as claimed in claim 10,wherein said restricted supply of pressure liquid includes two fixedrestrictors in series between the relay working space and a pressuresource and said passage connects at one end to the junction of the tworestrictors.

12. A variable-displacement pump as claimed in claim 11, wherein saidmanually-operable valves are each connected for joint operation with amain opencenter valve, the open-center valves being connected in seriesin a flow passage carrying the pump delivery, operation of anymanually-operable valve to select a particular displacement setting forthe pump also closing the associated open-center valve to connect pumpdelivery to a particular load.

13. A variable-displacement pump as claimed in claim 12, wherein onelimit of pump displacement adjustment comprises minimum displacement andincludplacement to a particular load.

1. A variable-displacement pump or a variable-displacement motor havinga hydraulically-operated servo-motor capable of moving to adjust thepump or the motor displacement by the supply of liquid to or from aworking volume in the servo-motor under the control of a main selectingvalve having two settings in one of which the working volume is fed witha restricted flow at pressure to urge the servo-motor to onedisplacement limit and in the other of which the working volume isconnected to low pressure to facilitate servo-motor movement to theother displacement limit under a restoring force, an auxiliary valvemeans actuatable by the servo-motor at a position corresponding to aselected displacement between the limits, and an auxiliary selectingvalve co-operable with the auxiliary valve means and having two settingsin one of which the valve means provides a vent to a low pressure zonesuch that at the selected position when the main selecting valve is insaid one position, a substantial pressure change in the working volumeis caused to occur to the extent to arrest the servo-motor substantiallyin the selected position, and in the other of which the valve means isrendered incapable of carrying liquid flow.
 2. A variable-displacementpump or a variable-displacement motor having a fluid-pressure-operatedservo-motor capable of adjusting pump or motor displacement from onelimit to another limit by the supply of fluid at pressure to or from aworking volume of the servo-motor, a valve means actuatable by theservo-motor at a position corresponding to a selected displacementbetween the limits, and a selecting valve co-operable with the saidvalve means, said selecting valve having two settings, in one of whichthe valve means is rendered capable of controlling the pressure in theworking volume to arrest movement of the servo-motor substantially inthe selected position, and in the other of which it is renderedincapable of controlling the pressure.
 3. A variable-displacement pumpor motor as claimed in claim 2, wherein the servo-motor is of thepiston-and-cylinder type, and the valve means is a port in the cylinderitself opened or closed by movement of the piston relative thereto atthe position of the port.
 4. A variable-displacement pump or motor asclaimed in claim 3, including a plurality of valve means at spaceddisplacement positions between the two limits, each valve means having aselecting valve associated therewith, whereby the servo-motor may bestopped at the position of any one of the valve means by appropriateselection of the settings of the selecting valves.
 5. Avariable-displacement pump or motor as claimed in claim 4, wherein theworking volume is fed with liquid at pressure from a supply ofrestricted flow rate and the action of the valve means is to connect ordis-connect the restricted supply to or from a low pressure zone inaccordance with movement of the servo-piston.
 6. A variable-displacementpump as claimed in claim 5, wherein the delivery of the pump itself, fedthrough a restrictor, forms the said supply of restricted flow rate forfeeding to the said working space.
 7. A variable-displacement pump ormotor as claimed in claim 2, including a relay valve movable inaccordance with movement of the selecting valve for rendering said valvemeans operative or in-operative.
 8. A variable-displacement pump ormotor as claimed in claim 7 wherein the relay valve comprises a relaypiston slidable in a relay cylinder against spring loading, the relaycylinder includes a plurality of ports connected to the servo cylinderports, and the relay piston is adjustable against spring loading toselectively connect and dis-connect differing numbers of the ports bymeans of selective pressure adjustment under control of the selectingvalve.
 9. A variable-displacement pump or motor as claimed in claim 8,wherein the selective pressure adjustment means includes a plurality ofselecting ports in said relay cylinder uncovered progressively duringmovement of said relay piston to connect to a working space in saidrelay cylinder, a restricted supply of pressure liquid connected to saidrelay working space, and said selecting valve comprises a plurality ofmanually operable valves for selectively connecting said selecting portsto drain, the relay valve moving to a position to partially close aselecting port connected to drain by a manually operable valve in orderto adjust pressure in the relay working space to meet the spring load atthat position.
 10. A variable-displacement pump or motor as claimed inclaim 9, wherein the manually-operable valve means are connected inseries in a passage extending to drain and the selecting ports areconnected to said passage at positions intermediate saidmanually-operable valves.
 11. A variable-displacement pump or motor asclaimed in claim 10, wherein said restricted supply of pressure liquidincludes two fixed restrictors in series between the relay working spaceand a pressure source and said passage connects at one end to thejunction of the two restrictors.
 12. A variable-displacement pump asclaimed in claim 11, wherein said manually-operable valves are eachconnected for joint operation with a main open-center valve, theopen-center valves being connected in series in a flow passage carryingthe pump delivery, operation of any manually-operable valve to select aparticular displacement setting for the pump also closing the associatedopen-center valve to connect pump delivery to a particular load.
 13. Avariable-displacement pump as claimed in claim 12, wherein one limit ofpump displacement adjustment comprises minimum displacement andincluding one open-center valve connected to a manually-operable valvebut separately operable to close the flow passage and to connect pumpdelivery at minimum displacement to a particular load.